A Propagator

I sit across from that man
My father,
And we each hold a beer
And smile,
And the day is still bright
And young
But the conversation is dry
Already,
So he asks me how my studies are,
"What have you learnt?"
And just for a second I flush
But quickly respond,
"I can construct a Feynman diagram."
He nods in approval,
I grab a napkin and scrawl
Show fermions and bosons dancing in the vacuum,
And we talk in ideas and forget the maths,
And I begin to feel I understand,
Though the exam's been and gone,
And the conversation flows
Then the wind starts to blow
And the napkin takes flight,
Then we're sat here again,
So I clear my throat and ask,
"How's work?"